A central piece for full rim frames is known. It is customarily assembled together with two temples to form an eyeglass frame, the temples being articulated to the central piece with the assistance of hinges arranged on the endpieces. If not only the central piece but also the earpieces are made of plastic, the result is a plastic frame. Once the two lenses have been inserted into the central piece, each of the two frame rims of the central piece completely surrounds the corresponding lens around its entire circumference. Such a frame rim completely surrounding the lens around its entire circumference is customarily, and here as well, called a "full rim." Accordingly, an eyeglass frame whose central piece has full rims is known as a full-rim frame. One of the advantages of a full-rim frame is that the lenses are protected around their edges against damage from impact or the like.
The known central piece is customarily produced by pressing a plastic material in an appropriate molding tool. One plastic frequently used for this purpose is an epoxy resin sold under the name "Optyl." The shape of the known central piece is predefined by the molding tool. Owing to the characteristics of the plastics customarily used for the known central piece, in combination with the dimensions of the central piece, this piece is a relatively rigid body whose form is essentially unchangeable. The course of the inner contour lines of the two full rims determines the so-called lens shape, i.e., the lineaments or shape of the outer circumferential lines of the lens to be set into the full rims. The shape of the left lens and the shape of the right lens are as a rule not congruent. However, in conventional eyeglasses with a full-rim frame, they are always symmetric to one another with reference to a vertical line through the middle. Hence in ophthalmic optics it is customary not to distinguish the shape of the left lens from the shape of the right lens, but rather to consider these a single lens shape. In the course of producing a pair of eyeglasses, the lens shape defined by the full rims of the central piece of the eyeglass frame is imaged on two lens blanks in such a way that the lens shape matches the course of the inner contour line of the corresponding full rim, so that the full rims can accept and retain the lenses. Since the lens shape is predetermined by a known central piece of the type under consideration here, it is necessary to produce appropriately varying central pieces to fit various lens shapes.
Also known are rimless frames for eyeglasses, which have no frame rims. In such frames, the lenses are an integral structural component of the central piece of the frame. The endpieces and bridge of a pair of eyeglasses with a rimless frame are joined to the lenses by screws, adhesive or the like. The lens shape is not predefined in a rimless frame. Rather, lenses of different shapes may be assembled together to make different eyeglasses, using the same bridges and endpieces. But in a pair of eyeglasses with a rimless frame, the edges of the lenses are not protected. Furthermore, at the points where they are joined to the bridge and endpieces, the lenses are subjected to locally elevated mechanical stress, which results in an increased risk of breakage.
Furthermore known are half-rim frames which only partially surround the lenses. The central piece of such a half-rim frame has two partial rims, extending from the bridge to either endpiece, which receive the lens around part of its circumference. Around the other portion of the circumference of the lens runs a plastic filament which is anchored to the two ends of the corresponding partial rim with the assistance of anchoring mechanisms. Such a half-rim frame defines the lens shape, at least along the section of the lens edge engaged by the partial rim.